Radiotherapy has been used in the treatment of tumors, such as tumors in lung and abdomen. Implanted markers, such as radio opaque fiducials, have been used in radiotherapy for target localization. In particular, in radiotherapy, precise delivery of the treatment dose is crucial in order to maximize the ratio between tumor dose and normal tissue dose. To achieve this goal x-ray visible markers may be implanted in or near a tumor. It allows use of the projected marker trajectory in x-ray images for image-guided radiotherapy (IGRT).
Sometimes, a marker may not have the simple shape such as the gold cylinder or BB. One example of such marker is the thin flexible Visicoil™ markers that are implanted in or around a tumor in anatomical sites such as lung, liver, and pancreas for the purpose of IGRT. The main advantage of Visicoil™ is reduced invasiveness due to its small diameter and less chance of migration in soft tissue because they are designed to coil up and engage the surrounding tissue after being implanted in the form of a straight wire. However because of the variability of the surrounding tissue, instead of completely coiling up, the marker may assume an irregular and unpredictable shape after being implanted in the anatomical three-dimensional space. In some cases, robust tracking of the fiducial can be a challenge because it shows up in varying extended shapes in the X-ray projections acquired from different orientations.